(11) (12) (13) (14) (15)
osp. Med, 40,7'5 (1969). (6) G.Rasmussen and Laeyer, 32, 830 (1970). (7) G. J. Safford, P. 6. Leung, G. F. Doebblei-, G.W. Brady, and E. F. X. Lyden, ( 8 ) G. Brink and M. Fa1 , J. Phys. Chem., 77, 2108 (1973). (9) J. R. Scherer, M, K (10) F. Rallo, R. ~ o ~ a iind n ~P.~ Gilvestrani, , Thermochim. Acta, 1, 311
(16) (17) (18)
(19)
(1970),
J. J. Lundberg and J. Kenttamaa, Suom. Kemjs~~/eht;~, 33, 104 (1960). J. M. I;. Cowis and P. M. Toporowski, Can. J. Chem., 39,2240 (1961). S. A. Schichman and R. L. Amey, J. Phys. Gha", 78,98 (1971). K.4. Picker and D. J. Tomliflson, Trans. FaradaySoc., 87, 1302 (1971). R. T. Beyer and S. V. Letcher, “Physical Ultrasonics," Academic Press, New York, N.Y., 1969. D.E.Bowen, J. Chem.Phys., 51, 1115(1969). D. E. BOWefl, J. the" Phys., 59, 4686 (1973). J. H. Andreae, P. D. Edmonds, and J. F. McKellar, Acustica, 19, 74 (1965). A. Rahman and F. H. Stiilinger, J. Amer. Chem. Sac:., 95, 7943 (1973).
~ ~ i o n Research ~ ~ r i f Laboratory, ~~ U. S. Army Natick Laboratories, Natick, Massachusetts 0 1760 (ReceivedJune 24, 1974) ~
~casts~assisted ~ by ~ Natick i Laboratories ~ ~ t
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ailed exaniination of the one-electron reduction of aromatic nitrogen heterocyclic compo~ndsin was carried out using the technique of pulse radiolysis and kinetic absorption spectrophotometry. The c~~mpounds studied include pyrazine (Pz), pyrimidine (Pm), pyridazine (Pd), q?i~noxaline(Qx), ~ ~ ~ ~ tand ~ acridine. ~ a ~ The ~ ~ reducing z ~ n agents e ~ used were eaq-, (CN3)&OH and (CH3)&6)- radicals. The efficiency and the rate of electron transfer from the latter two radicals were correlated with the redox potentials of the aza-aromatic compounds. The radical anions of these compounds are very weak acids and ~ ~ ~fast d protonation ~ r g ~by water or by other proton donors to form the neutral monohydro radicals and the ~ ~ ~ h y d ~ o rcations. a ~ ~ c Based al on the determined transient absorption spectra of the intermediates, and
the biue S h ~ f t ~ ~f~the g absorption maxima of the neutral monohydro radicals compared to the dihydro Furthermore, the radical cations are relatively inert to oxygen while the neutral radicals ith k > IO8 M - l sec-l~
~
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Although c o i i s ~ i ~ amount e ~ ~ b ~of ~work has been done on the elec~rocheInica~ reduction of aza-aromatic compounds in water and o r ~ ~s0lvents~-7 t ~ ~ c and on the esr spectra of the radical ions avd neutral species produced from N-het . erocyclic c o m p o ~ , very i ~ little ~ ~ ~ work ~ ~appears ~ ~ to have been carried out on the electronic absorption spectra and physicochem~c~~~ proper ties of these intermediates (except for ~ y r i d i ~rtio\icalsl')). y~ The most studied compounds have been the diaza derivatives of benzene, naphthalene, and anthracene. 'IL'he intrinsic importance of these diaza compounds is their e3ence in many biologically important molecules, e 6: , rimidines, purines, pteridines, alloxazines, and i ~ i ~ a ~ o ~ e ~ ~ Presented below i s an investigation of the intermediates produced from the ~ n e " ~ ~ ~ e creduction tron of pyrazine, py~ i ~ ~ d ~~~ ny re~ ,d a z qinin ~ ~ n e ,aline, phthalazine, and acridine in aqueous solutions. he fast-reaction technique of pulse radiolysis WSRSused t o d e t e ~ m the ~ ~ transient e optical
absorption spectra of these relatively short-lived species, their decay kinetics, and their acid-base properties. The electronic spectra of the radical anions of some of these compounds have been reported at room ten~perature'~ and at 7?'K16 in organic solvents. Experimental Section The pulse radiolytic experimental conditions used have been described p r e v i o u ~ l y . ~ The ~J~ radiolysis of water produces eaq- (2.8), OH(2.8), and H(0.6) radicals, where the numbers in parentheses are the G values of the radicals formed. The one-electron reduction of the diazines was accomplished in two ways: (1) by direct reaction with eaq-. The oxidizing OH radical was scavenged by using tert- butyl al~ 0 h o l . lThe ~ p radical t-BuOH produced absorbs very weakly above -280 nm ,and does not give rise to the initial intermediates 0 b s e r ~ e d . lHowever, ~ this alcohol /3 radical appears to affect the subsequent decay reactions of the diThe Journal al Physical Chemistry, Val. 78, No. 25, 7974
2
P. N. Maorthy and E. Hayon
azine radicals (see more below). Typically, the heterocyclic c o ~ ~ p o were ~ n ~ ~s r a d ~ ain t e0.5-2.0 d M t- BuOH solutions saturated with argon (1 atm). (2) The one-electron reduction was also a c c o ~ ~ p ~ i by s ~electron ed transfer from an 01~ y ~ ~ r radical, o x ye.g., ~ (CN3)2cOH ~ ~ ~ ~ or (CH3)2CO-. Soiiated in the presence of N2O (1 atm) in order to s ~ a ~ ? ~eagi~ge K2G
eaw
+
N, t
with k 1 = E,.? x i o 9 M s l sec-1 (re ~ r ~ react ~ with u the ~ alcohol, e ~ e.g
and the (C WJ)&38H radicals formed subsequently react with the diazxnes ~~~3~~~~~~
t 13
---4-
aDR lor*D-) 3. CH,GO@N,(+B*) (3)
The ~ ~ ~ c e n i tof~ the a ~ diazine ~ o ~ sand alcohol were chosen such that (a) all the euQ- J>95%) react with N2O uia reaction I, and none (45%) with the diazines, (b) all the ON radicals react with the alcohol and none ( 6 3 % ) react with the diazines; IC) all the a-hydroxyalkyl radicals transfer an electron to the ~ ~ a ~ and ~ ~ none i e s (
